In the field of control circuits for electromagnetic door locks, there have been several problems, including (1) residual magnetism, (2) inductive kick-back and (3) relatively slow release. Residual magnetism is an effect which is always found in electromagnets. When power to the magnet is cut off, the magnet continues to hold with a percentage of its energized force. The level of the remaining holding force involves residual magnetism or magnetic remenance, and is most prominently a function of the material used in the core of the electromagnet and the armature. The presence of residual magnetism in a magnetic lock will tend to hold the door closed so that the door appears to "stick" when a person uses it after the magnet has been de-energized. A weak or handicapped person may not be able to open the door at all. Regarding the second problem, that of inductive kick-back, a large electromagnet, such as is required for a practical magnetic lock, uses a large coil; and when the magnetic field collapses on cut-off, a considerable reverse voltage kick-back will appear on the power wires. This reverse kick-back is characterized by a high peak, typically in excess of 1,000 volts when a 12 volt or a 24 volt dc circuit is employed, and the pulse involves considerable total power. Since magnetic locks are used as part of an electronic security and control system, this kick-back pulse can destroy semiconductor devices located elsewhere in the control system. The pulse can also result in the application of a substantial shock to persons who happen to e touching or working on the circuits at the time the electromagnet is de-energized.
Concerning the third problem of release speed, an electromagnet of the size necessary for a practical magnetic door lock does not release instantaneously when the power to it is cut off. The magnetic field typically takes about 1/2 second to dissipate to its residual level following the cut-off of power. Magnetic locks are often used on emergency exit doors because they will not jam when power to them is cut off. With respect to the outside of the emergency exit door, no entry is possible because of the magnetic lock. However, on the inside, a mechanical switch is typically provided as part of the panic bar release mechanism, such that a person wishing to leave presses the panic bar, thus turning off the power to the magnetic lock. In an emergency situation it can be expected that a person would run at the emergency exit door. The one-half second release time would literally cause the person to re-bound from the door, possibly giving rise to the idea that the door was not usable. This of course could be life threatening in a case of a fire emergency. Ideally, a magnetic lock used on such an emergency exit door would release instantaneously such that a person could run at the door, depressing the panic bar, and could quickly get out.
Accordingly, principal objects of the present invention are to overcome these problems as outlined above, and avoid the sticking which is characteristic of residual magnetism, suppressing inductive kick-back to avoid the damage of associated electronic equipment, and to greatly increase the release speed for electromagnetic door locks. A further object of the present invention is to provide such a circuit which is simple and inexpensive while still effectively solving these problems outlined hereinabove.